It means that the total load of lights and A/C blower motor exceeded the charging capacity of your generator. Most cars of this era had a 40 Amp fusible link and only a 36 Amp alternator to charge the battery. Everything was powered by the battery, with the alternator being a portable battery charger.
Modern cars reverse this thought process as there are more drains on the electrical system with added electrical loads (power windows, power seats, stereo, electric fuel pump, computers (there are 14 different processors in my 2011 Silverado truck) electric fans, etc.). Modern cars have a 120 Amp or larger capacity alternator that out puts it's rated power at only 2,200 RPM compared to 3,600 RPM for your alternator to max out on power generation. The car runs off the alternator with the battery being for extra heavy load coverage and to start the motor.
If you are tired of dim lights at the stop light and worrying about reserve power to crank the car at night if you turn it off at the gas station, then I suggest upgrading your alternator with a new 120 Amp Delco-Remmy CS alternator that bolts on AND your car's wiring.
The wiring is old, the copper wire and it's PVC insulation are brittle. You don't need the insulation flaking off and starting a fire; or the extra load you added to the car causing a wire to over heat. The wire gauge chosen by GM was the absolute minimum it could get by with to save a few pennies per car. It was barely adequate when new and is inadequate to handle any additional power.
I strongly recommend upgrading the wiring if you increase the power load or alternator.